Mast Angles

[Kevin-the-lubber]

Hello all, I've been working on 3D modelling masts for the Cutty Sark in recent weeks and noticed something odd on the Campbell ship plans and would appreciate some advice. I'll explain the question through two drawings;

 

1. I had assumed that the different mast sections (lower, topmast, topgallant) would all be in line with each other, as shown below;

 

 

 

However, the Campbell drawings suggest the topgallants in particular are raked further than the lower and topmasts;

The Campbell drawings are more illustrations than technical plans, so is this just artists error or did the rake indeed vary? Incidentally, I know the Campbell angles are slightly wrong, it should be 86', 85' 84' fore, main mizzen respectively.

 

 

 

[VitusBering]

I looked through the Longridge books and the issue doesn't seem to be addressed directly. In Vol. 2 chapter 2 he does mention that Kipping's "Treatise on Masting and Rigging" has details from which most of the measurements and design elements Longridge uses are drawn. He also says it is not an exact science, that commercial vessels varied from builder to builder in masting and rigging elements based on experimentation and experience.

 

If I had to guess, and I can certainly do no more than that, I'd say the variance was unintentional in the drawings.

[allanyed]

Have you visited or contacted RMG about this?   research@rmg.co.uk  

Allan 

Edited March 1, 2023 by allanyed

[Kevin-the-lubber]

Thanks for your replies. Allan, no, in fact that hadn’t occurred to me so thanks, that’s a good suggestion. I was thinking that maybe it was a norm in tall ships, to possibly counter the increased flex you’d have all the way up there, but googling and searching on here hasn’t thrown up anything to back that up, so I’ll just assume it is indeed an artistic or printing error and keep them in line.

 

But having started this train of thought, it’s got me wondering as to how much difference the rake angles made anyway. I’d have thought that the masts would bend forwards to varying degrees under the wind pressure. I imagine all of this was a somewhat inexact science that was sometimes beneficial and at other times, not.

[mtaylor]

9 hours ago, Kevin-the-lubber said:

But having started this train of thought, it’s got me wondering as to how much difference the rake angles made anyway. I’d have thought that the masts would bend forwards to varying degrees under the wind pressure. I imagine all of this was a somewhat inexact science that was sometimes beneficial and at other times, not.

I would think that the shrouds would prevent mast movement under sail.  But, rope does stretch so maybe this is somewhat of a fail safe kind of thing.

[Kevin-the-lubber]

Spyglass, that’s interesting and makes a lot of sense. The stays on the Cutty Sark are wire hawsers, for reasons which are now obvious when reading your post. I think I kind of understand the logic of raking the masts - I assume you’re trying to get the wind to fill / flow down to the bottom of the sail, as close as possible to the hull, but I’ve also read of masts being raked forward, which makes less sense as that must make the bow ‘dig in’. 
 

Either way, I’ve settled on the rake angles given by Underhill and kept them in line.

[Jaager]

The captain and the sailing master were not passive cogs in their sailing machines. 

For exact angles of rake,  you would probably have to exhume one of these officers and ask them.  Even then, you would probably have to specify the exact date,  because there was probably fine tuning based of the season.   For some of this, Horse Shoes and Hand Grenades is sufficient.

[wefalck]

I think your original question was about a difference of rake between masts, top-masts and topgallant-masts ?

 

The answer is probably that there shouldn't be a difference. If there was, somehow the tops and the caps are not properly matched. However, a slight difference is not really consequential from a sailing point of view, only perhaps from an aethetic one.

 

The actual rake was very much subject to fashion and tradition. For instance, it became fasionable to have extremely raked masts around the 1840s, but by the end of the century the became basically vertical. The tradition is that the rake increases from front to back. On the other hand, there are many vernacular craft that had extreme rakes in both directions and often on the same craft - think of certain Portuguese or Chines boats.

 

For a given sailplan the actual rake of the mast will move its centre of gravity slightly, but taking in or setting sails probably has a more profound effect on a square-rigged ship. The situation is somewhat different on sloop-rigged yachts, which carry most of the time the fore and the main - there, changing the rake changes the tendency to turn leeward or otherwise, giving more or less pressure on the rudder/helm, and changes the turning behaviour when going through the wind.